The display of images on display devices such as a cathode ray tube (CRT) or a liquid crystal display (LCD) is a known art. The problem of luminance flashes or luminance jumps on LCDs is caused, at least in part, by differences between the rise and fall rates of pixel luminance changes. It is also known that the rise and fall rates of LCD pixel luminance changes are affected by a number of factors including, for example, the initial and final color state (e.g., image content), LCD type, manufacturing process variation, temperature variation and viewing angle. Further, it is also known that human eye sensitivity to the luminance jumps on LCDs varies with each individual.
The pixel luminance rise and fall rates of an LCD may differ due in part to the physics of liquid crystals. Therefore, for any particular pixel luminance transition, either the pixel rise rate may be faster than the pixel fall rate or the pixel fall rate may be faster than the pixel rise rate depending upon the manufacturer's design.
A scrolling image on a display, such as a sonar waterfall image, may exhibit substantial flicker with each scroll step of the image. A typical sonar waterfall display contains random noise displayed as gray scale data. When the image is scrolled, a large number of pixels may be changing from light to dark at the same time that a large number of pixels are changing from dark to light. Differing rise and fall rates during these complementary pixel transitions may result in discernible but unexpected and undesirable transient luminance variations or flashes, also referred to as flicker.
The LCD industry has typically been driven to minimize pixel response time which is defined as the sum of the rise and fall times. Therefore, there seems to be little motivation to match the pixel rise and fall rates, although it may be technically possible, because the matching of the pixel rise and fall rates could increase the pixel response time. The resulting flicker problem, due at least in part to the differing rise and fall rates, does not seem to affect enough users of LCDs to influence the decision to minimize pixel response time rather than match rise and fall rates.
U.S. Pat. No. 6,359,663 entitled “Conversion of a Video Signal for Driving a Liquid Crystal Display,” issued Mar. 19, 2002 to Gadeyne et al., and U.S. Pat. No. 6,909,472 entitled “Conversion of a Video Signal for Driving a Liquid Crystal Display,” issued Jun. 21, 2005 to Gadeyne et al. describe a method and apparatus for conversion of one input video signal to a second output video signal where the second video signal is modified from the first video signal to substantially match the luminance rise and fall times in shape and amplitude though inverted in slope for the LCD.
One drawback of the Gadeyne et al. apparatus and method is that this method is applied to all pixel elements within the LCD display. By slowing all pixel transition times to the slowest pixel transition time, the Gadeyne et al. method may cause smearing and loss of contrast when pixel changes happen faster than the slowest pixel transition. This smearing and loss of contrast will happen over the entire display instead of being isolated to a single data display window used for, for example, the display of sonar waterfall data. Therefore, motion video, such as camera video, played in a separate window on the display would receive potentially undesirable smearing and contrast loss.
Another drawback to the Gadeyne et al. apparatus and method is that a different complex implementation to substantially match the luminance rise and fall times in shape and amplitude is necessary to provide compensation for different specific display devices. As such, flicker compensated display of a sonar waterfall, for example, is limited only to those devices for which a specific complex implementation has been provided.